Human 8-oxoguanine-DNA glycosylase (OGG1) is the major enzyme for repairing 7-8, dihydro-8-oxoguanine (8-oxoG), a pre-mutagenic guanine base lesion produced by reactive oxygen species (ROS). The mutagenicity of 8-oxoguanine lies in its propensity to mispair with adenine during DNA replication. The importance of 8-oxoguanine and its repair by OGG1 are underscored by the frequent absence of the OGG1 allele in human lung tumors and the increased incidence of lung tumors in mice lacking a functional OGG1. 8-oxoguanine can occur in DNA by the oxidation of guanine in a G:C pair and by the incorporation of 8-oxoG into the newly synthesized nascent strand opposite cytosine or adenine during DNA replication or repair synthesis. Mispairings of 8-oxoG, when repaired by OGG1, could fix mutations if 8-oxoG in the parental strand is removed from a mispair with adenine. Accordingly, OGG1 should act only to remove 8-oxoG formed in DNA in situ and newly incorporated 8-oxoG in the nascent strand. If 8-oxoG in the parental strand becomes mispaired during DNA replication and is subsequently removed by OGG1, a G to T transversion mutation could result. Using co-immunoprecipitation, we identified an interaction between OGG1 and proliferating cell nuclear antigen (PCNA). PCNA is a multi-functional protein involved in DNA replication, repair synthesis and cell cycle regulation. The interaction of OGG1 with PCNA is of particular interest because known PCNA-binding proteins, such as DNA polymerases and components of the mismatch repair system, perform their functions on newly synthesized DNA are directed to the nascent strand via a directional interaction with PCNA. Using an in vitro binding assay and mutant OGG1 proteins, we have identified a functional consensus PCNA binding motif in the C-terminus of OGG1. Additionally, using immunofluorescence, we have shown that OGG1 and PCNA co-localize at sites of DNA synthesis in vivo. The association of OGG1 and PCNA suggests a bimodal mechanism of OGG1-mediated repair of 8-oxoguanine. In non-dividing cells, OGG1 and perhaps other DNA repair proteins may indiscriminately remove 8-oxoguanine as it occurs in DNA. During replication however, the OGG1-PCNA interaction may serve to direct OGG1 to the nascent strand in order to prevent fixation of mutations in the parental strand. The functional consequences of the interaction of OGG1 with PCNA, which are likely to be highly significant in vivo, are currently being investigated.